In carbon nanotubes, generally, one with the smaller number of layers has a high graphite structure. Therefore, it has been known that single-walled carbon nanotubes are high in characteristics such as electrical conductivity and heat conductivity. On the other hand, multi-walled carbon nanotubes have a low degree of graphitization, thus it has been also known that they have generally lower electrical conductivity and heat conductivity than single-walled carbon nanotubes. Since double-walled carbon nanotubes have both characteristic of single-walled carbon nanotubes and characteristic of multi-walled carbon nanotubes, there have been drawn attentions as a promising material in various applications.
In recent years, in chemical vapor deposition method (Patent document 1), plasma method (Non-patent document 1), pulse arc method (Patent document 2) and so on, it has been now known that aggregates of carbon nanotubes with high ratio of double-walled carbon nanotubes can be synthesized.
In the aggregates of carbon nanotubes produced, catalyst metal and impurities other than carbon nanotubes such as amorphous carbon and particulate carbon are mixed, and thus in order to bring out an intrinsic characteristic of carbon nanotubes sufficiently, an operation to remove the catalyst metal and carbon impurities becomes necessary.
For removing carbon impurities, generally a heating method in a gas phase is often used. For removing catalyst metal, it is common to use an acid. Although using a strong acid makes removal of catalyst metal easy, in the case of using a strong acid, carbon nanotubes are damaged and the characteristic deteriorates. Thus, practically it is necessary to use an acid having a relatively mild reactivity as an acid to be used for removal of catalyst metal. Non-patent document 2 describes that when single-walled carbon nanotubes are treated in a nitric acid solution, functionalization and defect of graphite structure take place. Non-patent document 3 denotes that when heating of multi-walled carbon nanotubes is continued, functionalization proceeds, G/D ratio in a Raman spectrum being one index showing a purity of carbon nanotubes is lowered. In a method shown concretely in Patent document 1, it is also reported that layers more than 20 layers are removed with nitric acid (which is understand on the basis of calculation from the average diameter before and after treatment, provided that interlayer distance of a carbon nanotube is 0.34 nm).
When removal of catalyst metal is only purpose, any acid which dissolves catalyst metal may be used, in general, when an acid such as nitric acid or a mixed acid of nitric acid with sulfuric acid is used, there is a fear that the surface of a carbon nanotube is functionalized, thus from the above-described reason, practically when the metal can be removed with hydrochloric acid, it is very often to use hydrochloric acid. In particular, in the case of a single-walled carbon nanotube, a graphite layer is constituted by only one layer, thus it comes under the notable influence of functionalization.
In order to increase electrical conductivity as the aggregate of carbon nanotubes, there have been devised a method that high conductive metallic carbon nanotubes are separated from semiconductive carbon nanotubes by electrocataphoresis, a synthesis method that metallic carbon nanotubes become major in a synthesis stage and the like, but they are techniques hardly applicable to carbon nanotubes with the number of layers of 2 or more, and the present situation is that double-walled carbon nanotubes having both merits of single-walled and multi-walled ones have not been obtained.    Patent document 1; Japanese Unexamined Patent Publication No. 2006-335604    Patent document 2; Japanese Unexamined Patent Publication No. 2004-168647    Patent document 3; Japanese Unexamined Patent Publication No. 2005-154200    Non-patent document 1: Journal of Physical Chemistry B, 107(2003), 8794-8798    Non-patent document 2: Journal of American Chemistry Society, 126(2004), 6095-6105    Non-patent document 3: Carbon 43(2005), 3124-3131